3.24.53.5 gamma correction

For gray mappings gamma correction can be turned on by ‘set palette gamma
<gamma>‘. <gamma> defaults to 1.5 which is quite suitable for most
terminals.

For color mappings no automatic gamma correction is done by gnuplot. However,
you may easily implement gamma correction. Here is an example for a gray
scale image by use of explicit functions for the red, green and blue component
with slightly different values of gamma